The invention relates to a device for cleaning the cylinders of a printing machine according to the preamble of claim 1. Here, this device comprises a driven washing roller, which can contact a cylinder to be cleaned. The dirt, which the washing cylinder has removed from the cylinder to be cleaned, is scraped off with a doctor blade, which at least temporarily engages the surface of the rotating washing roller. The doctor blade is mounted to a collecting trough, in which the scraped off dirt is collected and guided away.
Cleaning devices of this type are used for removing deposits and contaminants on cylinders of printing machines, which particularly develop by the intensive contact with the materials to be printed. When paper is printed, primarily paper dust and ink adheres to the cylinders of the printing machine.
In EP-A-1 106 354 an example for a known cleaning device is described. The washing roller, embodied as a brush roller and rotated by the motor of the device, is made to contact a cylinder, which in particular can be an offset-blanket cylinder, a counter-pressure cylinder, a plate or form cylinder, or a cooling roller, color roller, or wet roller of a printing machine. Simultaneously, the cleaning liquids are applied on the washing roller, so that the contaminants adhering to the cylinder are moistened and lifted off the cylinder using the rotating washing roller. The dirt accepted by the washing roller is scraped off by a doctor blade, which sits in several doctor blade holders in a clamped manner. They are inserted into a holding profile serving as a frame of the cleaning device and are fastened there. Below the doctor blade, a collection trough is provided.
Usually, the rotary direction of the brush roller is selected such that its surface travels a path as short as possible between the surface of the cylinder to be cleaned and the doctor blade. Furthermore, the brush roller is scraped off at the doctor blade in a commonly known fashion by elastically bending the individual bristles at the doctor blade and subsequently elastically reerecting the bristles, causing the dirt particles not only to be scraped off the doctor blade but also to be slung off the bristles during the reerection. For this reason, the rotary direction of the brush roller is commonly selected such that the direction that the dirt particles are slung off faces away from the cylinder to be cleaned.
Due to the fact that the contaminants accepted by the washing roller are provided with a strong adhesion force, in particular caused by the ink contained therein, i.e. they are sticky, in such a known cleaning device, some dirt always remains stuck to the holder of the doctor blade, causing dirt to build up with increasing time of operation. Such a dirt build-up can be avoided in that the doctor blade is directly mounted to the collection trough rather than to a separate carrier. An example for such a cleaning device is known from DE-A-197-02-082.
Directly scraping off dirt, removed from the cylinder surface to be cleaned by the brush roller, is not always optimal, though. Dirt particles are not the only matter slung off the brush roller but also the cleaning liquid accepted by the brush roller. During a most effective use of the cleaning liquid for cleaning cylinders in a printing machine it can be advantageous for the brush roller to be without contact to the doctor blade for a certain amount of time. Accordingly, the cleaning liquid remains at the surface of the brush roller and then after one rotation acts again on the surface of the cylinder to be cleaned. The amount of cleaning liquid used is considerably lower in a cleaning cycle without a doctor blade.
This is particularly important in heat-set printing machines, in which the freshly printed material enters a dryer. Such a dryer operates with heat, and the commonly used cleaning liquids contain a high content of volatile hydrocarbons. Particularly heat-set printing machines generally require for the cleaning of the cylinder surface to occur during production. Here, the material being printed, running through the printing machine, accepts the cleaning liquid and subsequently transports it into the dryer. The volatile hydrocarbons evaporate in the dryer, and additionally they are heated, so that there is a constant danger for the hydrocarbon concentration in the dryer to rise above a limit prescribed for safety reasons. Due to the fact that particularly in heat-set printing machines simultaneously materials of high value are to be printed, attention must be paid to the cleaning of the cylinder surface, during which waste paper is unavoidable, to be performed as quickly as possible.
However, these two requirements contradict each other: for the purpose of a high safety against an excess concentration of volatile hydrocarbons in the dryer it is necessary to use as little cleaning liquid as possible. However, in contrast thereto the requirement to keep the cleaning cycle as short as possible demands the use of relatively much cleaning liquid, because the time is too short for the contaminants on the surface of the cylinder to be cleaned to be moistened. The goal is therefore to achieve maximum efficiency in the use of cleaning liquids.
This includes for the doctor blade not to be kept in permanent contact with the surface of the washing roller but to interrupt this contact temporarily. Then, the cleaning liquid remains on the surface of the washing roller for a longer period of time and thus it has more time to soften the contaminants on the surface of the cylinder to be cleaned. Then it is necessary less frequently to add new cleaning liquid; the consumption of cleaning liquid drops accordingly while a comparable cleaning result is maintained.
One example for a cleaning device with a washing roller contacting the doctor blade, when necessary, and separable therefrom is found in U.S. Pat. No. 5,010,819. Here, the doctor blades are brought into the area of the washing roller by way of individually movable holders and, if necessary, made to contact the surface of the washing roller.